Spark Resistant Fans and Blower

The AMCA standard provides the system designer with a uniform way to specify the system requirements and provides fan manufacturers with general guidelines. The fan manufacturer must still develop unique designs to deal with the physical and practical limitations of fan equipment when developing construction methods to comply with AMCA.
Fan applications with airstreams of explosive or flammable particles or gases require spark-resistant system components for the safe handling of such airstreams. This includes components such as ductwork, dampers, filter devices, heating or cooling coils, and fans. This article presents practical considerations and methods of providing fans with varying types of Spark-Resistant Construction (SRC). The Air Movement and Control Association (AMCA) established a standard set of Classifications for Spark-Resistant Construction.
AMCA Type A Spark Resistant Construction requires all parts of the fan in contact with the air or gas being handled shall be made of nonferrous material. Steps must also be taken to assure that the impeller, bearings, and shaft are adequately attached and/or restrained to prevent a lateral or axial shift in these components.
In AMCA Type B Spark Resistant Construction the fan shall have a nonferrous impeller and nonferrous ring about the opening through which the shaft passes. Ferrous hubs, shafts, and hardware are allowed, provided construction is such that a shift of impeller or shaft will not permit two ferrous parts of the fan to rub or strike. Steps must also be taken to assure that the impeller, bearings, and shaft are adequately attached and/or restrained to prevent a lateral or axial shift in these components.
AMCA Type C Spark Resistant Construction requires the fan shall be so constructed that a shift of the impeller or shaft will not permit two ferrous parts of the fan to rub or strike.
It should be noted that:
1) No bearings, drive components, or electrical devices shall be placed in the air or gas stream unless they are constructed or enclosed in such a manner that failure of that component cannot ignite the surrounding gas stream.
2) The user shall electrically ground all fan parts.
3) For this Standard, nonferrous material shall be any material with less than 5% iron or any other material with demonstrated ability to be spark resistant.
4) The use of aluminum or aluminum alloys in the presence of steel which has been allowed to rust requires special consideration. Research by the U.S. Bureau of Mines and others has shown that aluminum impellers rubbing on rusty steel may cause high-intensity sparking.
The use of the above Standard in no way implies a guarantee of safety for any level of spark resistance. “Spark-resistant construction also does not protect against ignition of explosive gases caused by catastrophic failure or from any airstream material that may be present in a system.”
A major limitation is the practical availability of truly “nonferrous” alloys that really can be used in fan construction. There are certain alloys or noble metals than are truly nonferrous, alloys that contain no iron, but for the most part they are extremely expensive and/or difficult to obtain in forms and strengths necessary for fan construction. For most purposes, the fan manufacturer uses more readily available alloys that are considered nominally nonferrous and which have strength and work properties suited to fan construction.
Aluminum is the most frequently used alloy due to its low cost. However, as pointed out in the AMCA Standard, when aluminum is in close proximity to steel, careful maintenance programs are necessary to prevent rust, because aluminum rubbing against rusty steel can cause high-intensity sparking. In applications where such maintenance is not possible, an SRC method that places steel in the airstream is not recommended.
In particularly hazardous applications, the location of the fan and perhaps the entire system should be a major consideration. In some cases, protective enclosures around the fan or other mechanical parts in the system may be another protective step to lessen the danger in the event that a spark might occur in spite of the precautions taken. The system designer is in the best position to weigh the alternatives and specify the required fan equipment.
For additional information please refer to http://www.buffaloblower.com/roofexhauster/index.html.
Oleg Tchetchel
Fan and Blower Design Specialist
Buffalo Blower Co.
buffaloblower@buffaloblower.com
http://www.buffaloblower.com/supplyfan/index.html
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